When we discuss the six internal sensory faculties (cakkhu, through mano), they are INDRIYA when no “san” (lobha, dosa, moha) involved. That involves only “phassa.”
– When sensory faculties are used with a defiled mind, they become AYATANA. That involves “samphassa” or “san” + “phassa.” That rhymes as “samphassa.”
See, “How Do Sense Faculties Become Internal Āyatana?”
That is the easiest way to remember. But INDRIYA can be many types in other contexts.
Regarding the Vipaka PS:
chaṭṭhāyatana is the sixth ayatana or manayatana.